The problem with changing diff ratios is that you change rpm's all across the speed range for every gear. If you raise the 5th then you only change top end. I think he really just wants the extra leg at highway speeds, not at low speeds. Money wise your never gonna actually save anything by modding unless you keep the truck for 15 years.

Sold Truck: 89 DLX extra cab 4x4 22re Automatic/ New truck 97 T100 4x4 auto Sr5